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Old 12th April 2007, 03:30 PM   #1
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Talking MOSFET 'slightly high frequency' power amp

Hi all,

Very new to this game, (at least this end of the frequency spectrum). Here is my problem. I find myself trying to design an amplifier that doesnt quite fit the usual bill. Let me explain.

I need an amp that will work at and around 100KHz -> 150KHz, as I say not the usual audio style. It also needs to be very efficient because its going to be run from car battery +12V, in the field and sit there for a while.

Class-D springs to mind, but of course most sane people out there are making amps that output things people can hear!!!

The amp does NOT need to be particularly 'good' i.e. THD is not too big an issue, especially since I can't hear it.

The output of the amp will drive an inductor / arial and produce an RF output signal.

Would love the circuit to be 'power tunable' i.e. be able to dictate how much energy I drive into the coil depending on where it was sat.

It would be nice if component count was low, again one thinks of using say a IR2011 to take some of the donkey work out.

Cost is not really too big an issue.

So sounds easy, but can't find anything like it our there.....

Anyone got any suggestions ?

Dave Bray
RF specialist
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Old 12th April 2007, 03:34 PM   #2
fredos is offline fredos  Canada
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Class BD at your frequency with small filtering inductor!

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Old 12th April 2007, 03:37 PM   #3
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Would this be for communication with cavers?
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Old 12th April 2007, 03:42 PM   #4
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To add to my previous post.

Signals in this range may be receivable a few feet underground, so I wondered if the intention was to drive a frequency modulated carrier at 120kHz or so, with a simple receiver underground using a ferrite rod antenna and a PLL FM demodulator.

I guess not, as you wouldn't need a linear amplifier for that.
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Old 12th April 2007, 03:53 PM   #5
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If just a carrier is needed then a class-c amp is the best solution.


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Old 12th April 2007, 03:54 PM   #6
Did it Himself
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What output power/voltage swing do you need? That will dictate quite a lot!
__________________ my website for UK diy audio people - designs, PCBs, modules and more.
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Old 13th April 2007, 12:19 PM   #7
fumac is offline fumac  China
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canu tell us more info, perhaps my MCD can help u

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Old 13th April 2007, 02:28 PM   #8
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there is now a little slice of the VLF spectrum for experimental use in the US -- perhaps in the UK as well -- if so there may be an article on such a device in the RSGB magazine "RadCOM".
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Old 13th April 2007, 02:41 PM   #9
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In my homecountry this is 135.7-137.8 kHz for instance but others have priority using this range.


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Old 15th April 2007, 08:39 PM   #10
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I am overwelmed by the amount of response, its fantastic.

A little more information is required here.

What I am trying to do is build an antenna as you all quite rightly guessed. This would be fed a very narrow band frequency 134.20KHz and I want that transmitted as energy into free space.

The other end of this is a tiny transceiver which is 'charged' up by this pulse and replies back to a receiving station.

If anyone is interrested the tiny transceiver is made by Texas Instruments and is an RFID component.

A link to the tiny transceiver module below.

The trick with these little guys is to send out a pulse say 30mS long of energy at exactly the right frequency which will charge up the tiny internal capacitor encased in the chip.

As I mentioned before, the unit needs to be 12V (car battery) powered because it is going to sit in a field. Energy efficiency is a must as even a car battery has a limited amount of energy.

Of course this means the amp can be highly specialised, it only actually needs to amplify frequencies in a very narrow range.

As you might imagine TI do a single chip solution with a full bridge amp in it but it is very small and produces a range of only a few centimeters. I need something that will reach perhaps a meter.

As for spec, well the TI chip produces a 134.20KHz square wave which it feeds into a tuned LC circuit, (The L being the antenna naterally). Taking a scope and measuring some voltages I see with the tiny driver approx 100V across the antenna and a drive current around the 500mA.

Doing some math I calculate to reach say 70cm with the r3 law I would need 2.5A of drive at 250V. But I am certianly no EMR expert.

Once again thanks for all your replies, Ill post a circuit diagram of my 'prototype' with the next posting they may help to answer some more questions you all no doubt have.

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